1. Field of the Invention
This invention relates to methods and apparatus for avoiding undesirable growths, deposits and other formations in crystal growing operations; and, more particularly, to methods and apparatus for avoiding formation of projecting whiskers of silicon monoxide on the inner walls of a melt-containing crucible in Czochralski crystal growing operations.
It is conventional to provide single crystalline forms of many solid materials by preparing a melt of the material and contacting the surface of the melt with a previously prepared seed crystal of the material of the desired crystalline lattice orientation. The seed crystal is withdrawn from the melt at a rate of the order of a few inches an hour while the crystal and the melt are counter-rotated with respect to each other.
Typically, the chamber in which the crystal is grown is first partially evacuated and then backfilled to a positive pressure with a continuing flow of a gas, such as argon, which serves as the ambient during the crystal growth. The positive pressure aids in avoiding entrance of undesired contaminants into the system during the growth. With this described technique, commonly termed the Czochralski technique, crystals several feet in length and several inches in diameter are routinely grown by workers in the silicon semiconductor technologies.
Particularly in the silicon semiconductor technologies, the melt, which may be at an average temperature of 1420.degree. Centigrade, typically is contained in a quartz (silicon dioxide) crucible. At the temperature involved, reaction of the quartz crucible with the molten material occurs, and thus provides a source of oxygen, which in turn reacts with the molten silicon to produce silicon monoxide. The silicon monoxide is given off in vapor form from the melt and tends to preferentially grow, condense, or otherwise form dendrites, i.e., projecting formations, on what are apparently nucleation sites on the inner wall of the crucible just above the initial level of the melt surface during growth of the crystal.
These dendrites extend radially inward from the inner wall of the crucible, and often become dislodged and drop into the melt, where convection currents or other flow patterns carry them to the growing crystal. Upon impact with the growing crystal, the desired crystalline structure usually is lost and any further growth would produce undesirable material.
2. Description of the Prior Art
To alleviate the problems associated with silicon monoxide dendrites and other formations, a disclosure in an article entitled "Czochralski Silicon Crystal Growth at Reduced Pressures" by C. T. Chartier et al., in State Technology, 1975, pages 31-33, suggests growing crystals by the Czochralski technique in a vacuum, and alleges that significantly improved results are obtained. This vacuum technique is in strong contradistinction to the above-described conventional techniques for Czochralski growth, wherein the crystal growing chamber is purged with a positive pressure of a gas, such as argon, during the crystal growing process.
A distinct advantage of using a positive pressure of a gas is that it tends to minimize any likelihood of contamination entering the system during the time that it is under positive pressure. A related advantage from an operating viewpoint is that with a positive pressure system, a port in the system can be opened during the growth cycle, if desired for any of a variety of reasons, without incurring significant entrance of contaminants into the system.
These advantages of the positive pressure technique suggest a disadvantage of the vacuum technique, namely that entrance of contamination while the system is under vacuum, is rendered more likely in the event that any leak in the system should develop. A further apparent problem with the vacuum technique is the significantly more complex equipment required to perform the Czochralski process under vacuum. Still a further apparent potential problem with the vacuum technique is the possible adverse effects of the absence of the flowing gas upon the heat transfer dynamics of a Czochralski-type system.